1. Field of the Invention:
The present invention generally relates to a magnetic bearing system, and more particularly, to an unstable vibration prevention apparatus designed for an active type magnetic bearing system adapted to control a magnetic force by using an electronic circuit.
2. Description of the Prior Art:
A magnetic bearing is capable of suspending an object with a magnetic force in a completely non-contact state and therefore has a wide variety of features. Such magnetic bearings show the following advantages of virtue of the absence of mechanical contact:
(1) no abrasion is created, and hence the life thereof is semipermanent; PA1 (2) both vibrations and noises are small with no abrasion occurring; PA1 (3) rotational drag (a loss associated with rotation) is small, and high-speed rotation can be attained; PA1 (4) magnetic bearings are usuable under such special circumstances as operataion in a vacuum, at relatively high and low temperatures and in a radioactive atmosphere; and PA1 (5) a perfectly oil-free system is practicable due to the absence of any need for lubricants. PA1 (1) the stiffness and damping characteristics of the bearing are freely adjustable; PA1 (2) since a wide bearing-gap is capable of being set, the position of the rotor can be moved with minute precision; and PA1 (3) a displacement sensor can be built-in the system, which arrangement facilitates the monitoring of the system's operation. PA1 assuming two control axes perpendicular to the rotational axis of a rotary body; PA1 deciding a whirling direction of the rotational axis in one of the control axes; PA1 producing a signal indicating such a whirling direction; and PA1 adding this signal to a signal indicating a displacement of the rotational axis detected in the other of the control axes.
In addition, a control type magnetic bearing adapted to control a magnetic force has the following advantages:
In the conventional active type magnetic bearing system wherein an electronic circuit serves to control the magnetic force to adjust the radial position of a rotary body, instability of the rotary body becomes more marked as the rotational frequency of the rotary body increases. In the range of low stiffness and damping within lower frequencies, whirling (unstable vibratin) of the coniccal mode in which the direction of the whirling is opposite to the rotaational direction is in some cases generated. Morever, in the range of higher frequencies, whirling of the conical mode wherein the whirling is effected in the same direction as that of rotation is likewise produced if the damping force decreases due to internal damping or other causes. This kind of whirling will be explained more fully laater. Such unstable vibration is propagated to a casing via a fixed member on which the stator of the magnetic bearing is mounted, thereby causing resonance of the structure. This results in augmentation of the control current of the magnetic bearing, which in turn results in the generation of heat. In addition, the unstable vibration sometimes abruptly diverges and an uncontrollable state is thereby induced.
Such unstable vibration can be prevented by increasing the gain of a control system to heighten the stiffness of a bearing and increment the damping. In order to increase the gain, however, the capacity of control circuit must be raised. Such an increase in the capacity of the control circuit raises the problem of increased vibration during the rottion and excited resonance of the structure.
Since the above-described unstable vibration corresponds to the self-excited vibraion of the rotor, if the natural frequency causing an unstable vibration is set to be within a proper range at the design stage, such unstable vibration can be avoided. However, when a variety of constraints exist in regard to the structure of the rotary body and the like, in most cases it is difficult to arbitrarily set the natural frequency.